Microwave switch



Nov. 7, 1961 R. M. HILL ETAL MICROWAVE swxwcn 2 Sheets-Sheet 1 Filed Aug. 25, 1958 POWER 2? SUPPLY ANTENNA SWITCH TO TRANSMITTER NO. I

SWITCHING VOLTAGE TO TRANSMITTER NO. 2 26 INVENTORS M U M Y E L N M l O w j/A TIU R Y M m 8 Nov. 7, 1961 R. M. HILL EIAL MICROWAVE SWITCH Filed Aug. 25, 1958 RELATIVE BREAKDOWN POWER P IN DB RATIO OF FlELD T0 CYCLOTRON RESONANT FIELD BY 2 A 2 Sheets-Sheet 2 BREAKDOWN TIME-T IN secowos x 10 INVENTORS ROBERT M. HILL BAUM ATTORNEY United States Patent O 3,008,098 MICROWAVE SWITCH Robert M. Hill, Palo Alto, and Sidney J. Tetenbaum, Los Altos, C'alif., assignors, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed Aug. 25, 1958, Ser. No. 756,752 4 Claims. (Cl. 333-7) This invention relates to apparatus for the transmission and control of microwave energy, and more particularly to devices for selectively switching the flow of high power microwave energy in a branched transmission system.

High power microwave switching devices have a variety of microwave applications such as, for example, in a radar system for connecting either of two or more microwave transmitters to a common antenna, or conversely, for connecting two or more antennas to a common transmitter. For these and other applications, it is highly desirable that the time of switching be controllable, that the switch be fast-acting, in the order of a few milliseconds or less, and that it be capable of handling high powers, with peaks from several hundred kilowatts to a few megawatts. Other desirable features of such a switch are that it be capable of operating over a fairly wide range of frequencies and that it have a relatively high isolation characteristic, in the order of 50 db.

In the past, controlled switching of microwave power has been accomplished by mechanical switches which are relatively slow in operation. Other types of microwave switches employ ferrite elements which have limited power handling capabilities because of temperature dependence. Gaseous discharge devices are particularly suitable for rapid switching of high powers, the best example of this being the well-known transmit-receive tube in which a microwave gas discharge acts as the switching element. However, in this type of switch, the discharge is solely responsive to the magnitude of electromagnetic energy which is to be switched and is not otherwise controllable. In other words, the time at which switching occurs incides exclusively with transmission of electromagnetic energy above a certain intensity level.

It is therefore an object of the present invention to provide a high power microwave switching device which is capable of being switched on and off at any time during the transmission of electromagnetic energy above a certain power level.

Another object is to provide a switching device of this type which is capable of operating from a switch off to a switch on condition in a few milliseconds or less.

Another object is the provision of a switch capable of handling peak powers from several hundred kilowatts to a few megawatts.

Still another object is a provision of a compact microwave switch having an insertion loss in the order of 1 db.

A further object is the provision of a switching device which affords a high degree of isolation between components of the transmission system, in the order of 50 db or more.

A more specific object is the provision of a magnetically controlled resonance type gas discharge switch which is opened and closed by shifting the biasing magnetic field between a low or zero value and resonance intensity.

The foregoing objects, and others which will appear from the description to follow, are attained by the use of a principle involving the phenomenon of electron cyclotron resonance in the gas. This phenomenon is manifest when a suitabe ionized gas is subjected to a magnetic field of proper intensity and is excited by electromagnetic energy having a component of the oscillating electric field which is normal to the direction of the applied magnetic field. A sharp resonance in the breakdown power of the gas occurs when the static magnetic field B is related to the angular frequency of the propagated electromagnetic waves by the expression wherein m and e are the mass and charge of the electron, and B designates the electron cyclotron resonant magnetic field. Under such conditions, electrons in the gas spiral about the flux lines of the static magnetic field with an oribtal angular frequency equal to w, and between collisions with neutral gas atoms, can continuously absorb energy from the microwave electric field. The degree of such energy absorption is maximum when B=B,,. When B is not equal to B the orbital motion of the electrons is out of phase with the microwave electric field, and the energy absorbed from the electric field is substantially reduced. In other words, when the condition of electron cyclotron resonance exists, there is a maximum transfer of energy from the microwaves to the electrons in the gas, and the breakdown power of the device is a minimum. Conversely, a high level of power is required for a breakdown of the gas when the applied field B is not equal to B One example of a microwave switching circuit constructed in accordance with our invention is a dual transmission system with two high power microwave transmitters connected to the ends of two branch wave guides, respectively, the other ends of which are joined at an angle to opposite sides of a single wave guide trunk to which a load, such as an antenna, is connected. A suitable ionizable gas is confined in a switch tube in each of the branch lines at the place where these lines join the trunk line. Unidirectional or static magnetic biasing fields for the gas switches are provided by two separate electromagnets which are energized one at a. time by a power supply controlled by an electronic switch so as to produce a cyclotron resonant field in the gas of the first switch and an off-resonant or substantially zero field in the second, and vice versa. Only one transmitter at a time is in operation. When the transmitter connected to the second branch line is on, the gas switch in the second line is closed or unfired and permits passage therethrough of the microwave energy generated by that transmitter The power breakdown level of the gas in the second switch is very high, substantially greater than the source power, and therefore microwave energy propagates readily through the gas with minimum attenuation as though it were a closed switch. This same microwave energy, however, causes rapid ionization and ulti mate breakdown of the gas in the first switch, resulting in a large change in the impedance of the latter so that the microwaves cannot enter the first branch line but propagate down the trunk line to the load. The gas switch in the first line is said to be open since the first line is isolated by it from the power transmission. In another sense, the high density electron discharge in the first switch places a short across the entrance to the first line. Microwave energy coming down the second branch line sees this short as though it were a continuation of the wave guide wall, and continues past the first line to the load. In order that there shall be a clear understanding of terms used herein, a switch is closed when the gas is unfired, and is open when it is fired.

When it is desired to substitute one transmitter for the other in the system, the formerly inoperative transmitter is turned on and the other transmitter turned off. Simultaneously, the biasing magnetic fields on the two switches are interchanged and the substitution is complete. Control of the on-oif condition of the transmitters preferably is synchronized with control of the magnetic biasing fields for the gas switches so that both switching actions occur simultaneously. The transmitter which had been operating is not only turned off but is effectively isolated from the system.

"Other features of the invention will become apparent, and its construction and operation better understood from the following detailed description taken in connection with the accompanying drawings in which:

FIGURE 1 is a block diagram showing a transmission system with a 120 E-plane Y switch embodying our invention and connecting a pair of microwave transmittersto a common antenna.

FIGURE 2 is an enlarged fragmentary partially sectioned view of the switch together with the electronic control for the energizing current of electromagnets.

FIGURE .3 is a transverse section of the electromagnet and the wave guide comprising the microwave switch, the section being taken on line 33 of FIGURE 2.

FIGURE 4 is a plot of a series of curves showing the relative breakdown ,power in gas at different pressures and the breakdown time of the switch in accordance with the invention.

FIGURE 5 is aschematic drawing of a modified transmission system using two Y-type switches.

A microwave switching device constructed in accordance with the principle of this invention has utility and advantage in a radar system represented in block form in FIGURE 1 and comprising an antenna connected by a trunk microwave transmission line 12 to branch transmission lines 13 and 14, each of which diverges at an angle of 60 degrees from the axis of line 12 to form a Y junction. It will be understood that any hollow microwave transmission line may be used in the practice of this invention, and we have illustrated conventional rectangular wave guide as an example of such a line. The opposite end of branch line 13 is connected to a high power electromagnetic energy transmitter 16, such as a magnetron, and branch line 14 connects to the output of a similar microwave transmitter 17. While the present invention is concerned primarily with the transmission function of the radar system, there is also shown in FIGURE 1, in broken line, a receiver 18 and TR switch 19 connected by transmission line 20 to the trunk line .12. It will be understood, however, that the use of the invention is not limited to a radar system, but can be used with advantage and merit in any microwave transmission system where a high degree of isolation of and rapid switching between high power sources are desired.

The purpose of providing a radar or other transmission system with two transmitters is to obtain a greater operating bandwidth for the system than is possible or achievable with one transmitter alone, and thereby permit selection of an optimum operating frequency. For example, transmitter 16 may be tunable over a frequency range of 2.60 to 3.2 kilomegacycles, whereas transmitter 17 operates over a 3.1 to 3.8 kilomegacycle range. In

.orderthat the total frequency range of 2.60 to 3.8 kilogized.

megacycles may be made available instantaneously for transmission purposes, it is essential that one transmitter be switched off and the other switched on in a minimum of time, in the order of a few milliseconds, so as not to interrupt the searching function of the system. To this end, microwave switches .22 and 23 are provided in lines 13 and 14, respectively. These switches operate on the gas discharge principle and include an ionizable gas, such as argon, which is pervaded by static magnetic fields generated by electromagnets 25 and 26. A power supply 27 is connected by an electronic switch 28 to each of the electromagnets 25 and 26 in such a manner that at any one instant energizing current is supplied to one electromagnet and is cut off from the other. The amount of energizing current supplied to the one magnet is predetermined so that the field created by the energized magnet causes the gas in the associated switch to be in cyclotron resonance with the electric component of microwave energy from the transmitter in the opposite branch line. For example, with transmitter 17 operating, switch 28 connects electromagnet 25 to the power supply so that the gas in switch 22 is magnetically biased to cyclotron resonance, and electromagnet 26 is de-ener- Microwave energy from transmitter 17 passes unaffected through the gas in switch 23 and causes a discharge in the gas of switch 22 which acts as a short circuit across the junction of branch line 13 with trunk line 12. This effectively isolates branch 13 from the system and all of the transmitted power is directed through wave guide 12 to the antenna. Conversely, when transmitter 16 is operating and transmitter 17 is not, electromagnet 26 is energized and the cyclotron resonant discharge occurring in the gas of switch 23 directs the energy around the bend to antenna 10.

It is understood that the biasing magnets may and preferably do comprise permanent magnets having fixed fields at less than resonant intensities, the additional field supplied by the electric coils being suificient to bring the total field to resonant intensity. It is not necessary that the biasing field be reduced to zero in order to open the switch since the characteristic breakdown curve for the gas has a relatively steep slope, that is, breakdown power increases rapidly for small changes in applied field.

A preferred form of gas envelope or switch tube is shown in FIGURE 2 and comprises a section 31 of rectangular wave guide having longitudinally spaced transverse walls 32 and 33 sealed to the guide section and defining therebetween a gas chamber 34. Since the structures of the gas tubes for switches 22 and 23 are substantially identical, it will be sufficient to describe one of them, and like parts of the tubes are identified on the drawings by like numbers and their primes. End walls 32 and 33 lie in planes perpendicular to the guide walls and are fitted with broadband low-loss microwave windows 35 and 36, made of a suitable microwave permeable material. The longitudinal spacing of these walls of each tube is such as to enhance broadband operation of the tube. The gas in each of switches 22 and 23 is confined in the path of microwave propagation through branch lines 13 and 14, respectively, in such a manner that the electron cyclotron resonance discharge, when it occurs, effectively transforms the transverse wall of the one switch into a continuation of the wave guide wall of the opposite branch line. It is important, therefore, for the proper operation of each switch, that the resonance gas discharge occurs across the opening of the branch line at plane P (see FIGURE 1). The breakdown arc extends across the opening so that microwaves coming down the opposite branch line see a continuous conducting path around the degree bend in the guide. This result is achieved by the herein described switch tube with the planar window 33 forming, as it were, part of the outside wall of the guide.

Each of the electromagnets comprises a wire coil 38 wound around a core 39 of a permanent horseshoe type magnet having poles 40 and 41, see FIGURE 3, adjacent to the narrow walls of the wave guide section 3 1 as shown. The fixed field of the permanent magnet is perpendicular to the electric vector E of the propagated electromagnetic waves and has an intensity less than the resonant field. A direct current passing through the coil produces an additional unidirectional magnetic field which brings the total field to an intensity sufficient to produce electron cyclotron resonance in the gas. The cit-resonant field which exists when the coil is de-energized ettectively renders the gas insensitive to the power output of the transmitter. Since the switch tubes in the herein illustrated Y configuration are relatively close to each other, there is a tendency of flux from the magnetic field of one switch to leak across the gas in the other switch. To reduce this leakage, suitable magnetic shunts, not shown, may be included in the assembly in accordance with well-known practices in magnetic circuit design. It should be noted that a longitudinally directed magnetic field can be employed to bias the switch if desired, but such an arrangement requires careful control of the resulting magnetic leakage to insure independent operation of each switch. Eddy current losses in the wave guide walls as a result of rapid switching of coil currents are minimized by using thin-wall tubes, or by constructing the tubes with plated ceramic walls.

A feature of this invention is the rapidity with which power can be switched off in one branch leg and on in the other, this limit being defined by the speed with which the static magnetic field can be switched between resonance and cit-resonance values. In order to accomplish rapid switching action, the coils 38 and 38 of the magnets are connected by lines 41 and 42 to an energizing power source 27 through an electronic switch comprising tubes 43 and 44. These tubes may be triodes, as shown, whose control grids are connected by lines 45 and 46, respectively, to a switching voltage source, not shown, which causes one control tube at a time to conduct by means of the control of the bias voltages on the tubes. This source, for example, may be a square wave generator having its output voltage applied directly to one control grid and through a voltage inverter to the other. One tube is biased above cut-01f and conducts while the other is biased below cut-ofi and does not. When the square wave reverses its polarity, the bias voltages on the tubes are interchanged and the microwave switching action takes place. The switching volt-ages applied to the grids of control tubes 43 and 44 may be programmed or otherwise synchronized with the operation of the transmitters to insure that the resonant field is applied to one switch when the transmitter in the opposite branch of the Y assembly is turned on.

The operation of this microwave switching apparatus .will now be explained. When, for example, it is desired .grids of control tubes 43 and 44 such that tube 44 conducts and tube 43 does not conduct, thereby causing coil 38' to be energized and coil 38 to be de-energized.

The amount of current passing through coil 38' is such that the total magnetic field produces electron cyclotron resonance in the gas of switch 23. Electromagnetic waves generatedby transmitter 16 prop-agate through branch line 13, through the gas in chamber 34 of switch 22 and enter wave guide 12. These waves are incident upon the mangetically biased gas in chamber 34 of switch 23 and cause an almost instantaneous ionization and breakdown of the gas. The high intensity discharge appears as an arc across window 36 and effectively isolates branch line 14 from the circuit. Substantially all of the microwave energy from transmitter 16 passes around the 120 degree bend and through line 12 to the radiating antenna When it is desired to substitute transmitter 17 for transmitter 16 in the system, the former is turned on, transmitter 16 is switched 01f, and the switching voltages on lines 45 and 46 are reversed so that magnet 26 is deenergized and magnet 25 is energized to produce a resonant field. The output of transmitter 17 then causes a discharge across window 36 and branch line 13 is isolated from the system.

The principle upon which this invention is based is illustrated graphically in FIGURE 4 wherein the three solid line curves 50, 51 and 52 represent the relative breakdown power compared to applied field for a gas at three successively higher gas pressures. It will be noted that the power required to cause breakdown of the gas passes through a minimum value for a field intensity equal to B (cyclotron resonant field), and that the lower the pressure the narrower the resonance characteristic. At sufficiently low pressures, the power required for breakdown of the gas at cyclotron resonance can be four orders of magnitude less than the breakdown power with zero applied magnetic field. The time required for the gas to break down decreases to a value less than .02 microsecond at resonance.

Tests have been conducted on a gas discharge switch embodying the present invention and results have been obtained as follows:

Power switch:

Peak 250 kw. Average 250 watts. Pulse width 2 nsec. Frequency 2.85 kmc. High level VSWR 1.20 to 1. High level insertion loss 0.5 db. High level firing time 2 X l() seconds. High level isolation 60 db. Antenna bandwidth 2.7-3 .7 kmc. Transmitter 1 bandwidth 2.7-3.3 kmc. Transmitter 2 bandwidth 3.1-3.7 kmc.

From the foregoing description, it will be seen we have provided a high power extremely rapid microwave gas discharge switch which is capable of being controlled in accordance with conditions external to the transmitter circuit. The high power capabilities of the gas discharge switch make it ideally suited for modern high power transmission systems. The high degree of isolation afforded by the switch insures protection of components of the system. In addition, broadband operation of the system is readily achieved because the gas discharge switch of this invention is inherently a broader band component as compared to conventional gas switches with field concentrating projections, as in standard TR tubes.

The principle of the invention can readily be utilized in any branched microwave system wherein switching of microwave power to different branches is desired. As mentioned above, such a system may comprise a single high power microwave transmitter and two or more antennas, or one transmitter and two or more sections of one antenna. Such an arrangement is useful for lobe switching and for achieving different radiation characteristics practically instantaneously. By way of illustrating such an arrangement, consider in FIGURE 1 that 10 represents a transmitter and blocks 16 and 17 are antennas. Microwave energy propagates down line 12 from right to left as viewed, and, depending upon which switch is energized, will be directed to either of antennas 16 or 17. The advantages of rapid switching of the microwave power in such a system by use of the herein described switches are evident.

The switching apparatus described above requires that only one of the two transmitters be active at a time. The operation of the gas discharge switches is synchronized with the transmitter energizing circuit so that one switch and one transmitter only are operated at one time. However, it may be desirable to operate both transmitters simultaneously and continuously, and to depend entirely upon the microwave switches to direct the output power of one or the other transmitter to the proper load terminal. For this purpose, two or our V-configuration switches 60 and 61, each of which is identical to the Y-type switch of FIGURES 1, 2 and 3, are joined together as shown schematically in FIGURE The branches 62 and 63 of switch 60 contain magnetically biased gas tubes S and S and similarly branch arms 64 and 65 of switch 61 contain tubes S and S Trunk lines 66 and 67 of the switches connect to microwave transmitters T and T respectively. The inner branches 63 and 64 are joined in another Y-configuration to a common output line 68 which is connected to an antenna. The outer branch lines 62 and 65 are terminated in balanced loads L and L respectively. The field producing magnets and electronic switches associated with gas tubes S S and 5.; are omitted for the sake of clarity of the illustration.

In operation, both transmitters, T and T are active at the same time and operate continuously. In order to connect T to antenna A, the static magnetic fields on S and S are increased to produce electron cyclotron resonance in the gas therein, so that both S and S fire. S and 8,; are biased by off-resonant fields and are unfired. The output of T therefore is blocked by the discharge in S passes through the cold or unfired tubes S in line 63, is blocked by a discharge at window W in S from entering line 64, and passes through line 68 to the antenna. The output of T is blocked by the discharge at window W in S and passes into 11ine 6-5 through unfired tube 8.; and is absorbed by a balanced load L When T is to be connected to antenna A in place of T the magnetic biasing fields on S and 8;; are reduced substantially below resonance, and resonance fields are applied to S and S The output of T is now diverted from line 63 by the discharge at W of S into line 62 to load L The output of T causes a discharge at W of S and passes through line 64 and cold tube S into line 68. A discharge at window W of S blocks entry of this energy into branch 63 and it therefore passes to antenna A.

Additional transmitters may be included in the system to feed a single antenna by connecting as many Y-type switches in the manner shown and described, and thus considerable flexibility and versatility in transmitting are achievable. The electronic switches for the several electromagnets may be controlled manually, or automatically as by suitable control apparatus to provide programmed operation of the transmission system.

Although a typical embodiment of the invention has been shown and described, it will be understood that the invention is not limited to the specific apparatus herein described since various modifications can be made to it, and it is contemplated by the appended claims to cover any such modifications which follow the true spirit and scope of the invention.

A single Y-type switch may be modified to adapt it to control the outputs of two simultaneously operating transmitters. Such a modification includes a one-way power absorption element in each arm that is connected to a transmitter. These absorption elements, called microwave isolators, may be placed between the respective gas switches and transmitters and each functions to allow microwave power to pass from the associated transmitter in a direction only toward the antenna. For example, isolators 75 and 76, illustrated in phantom line in FIGURE 1, are inserted in arms 13 and 14, respectively, between the transmitters and gas switches. When transmitter 16 is connected to the antenna, switch 23 is fired, and a discharge across both windows 35' and 36' occur. The output of transmitter 17 is reflected by the effective short circuit at window 35 and is absorbed by isolator 76 before it reaches transmitter 17. Such an arrangement of isoiators might well be used even when operation with one transmitter at a time is desired because the isolators protect the apparatus from burnout due to inadvertent simultaneous operation of the transmitters.

What is claimed is:

1. In combination with a first microwave transmitter, a first microwave transmission line connected to said transmitter, a second microwave transmitter, a second microwave transmission line connected to the second transmitter, a load, and a third transmission line having sides and connecting said load to said first and second lines, the improvement of microwave switching apparatus comprising a first container of gas in said first transmission line at the junction of the latter with said third line, said container having a microwave permeable window traversing said first line and lying in the plane of one side of the third line, first electromagnet means adapted when energized to produce in said gas a magnetic field of sufiicient intensity to cause electron cyclotron resonance in said gas at the frequency of electromagnetic waves transmitted by said second transmitter, a second container of gas in said second line at the junction thereof with the third line, said second container having a microwave permeable window traversing the second line and lying in the plane of the other side of the third line, and second electromagnet means adapted when energized to produce in the gas of said second container a magneticfield of sufficient intensity to cause electron cyclotron resonance in the latter gas at the frequency of electromagnetic waves transmitted by said first transmitter, a source of power for energizing said electromagnet means, and switch means between said source and said first and second electromagnet means and operative to connect said source to one of said electromagnet means while disconnecting the source from the other electromagnet means.

2. In combination,three sections of hollow wave guide joined together in a Y configuration and adapted to transfer high frequency electromagnetic waves therethrough, a volume of ionizable gas confined in each of two of said sections at .the junction with the third, two separate magnet means for producing a unidirectional magnetic field through the gas in said sections, respectively, in a direction perpendicular to the electric component of the electromagnetic waves propagated through said sections, means for energizing each magnetmeans to produce a field having an intensity suflicient .to cause electron cyclotron resonance in said gas at the frequency of oscillation of the electric component of theelectromagnetic waves propagated through the opposite section, and switch means for selectively connecting said energizing means to one magnet means and disconnecting it from the other whereby microwave energy propagating through the wave guide section associated with the de-energized magnet means passes through the gas therein to said third wave guide section and simultaneously causes an electron discharge in the gas in said second section for electrically isolating the latter section from the other two.

3. The combination according to claim 2 in which the gas in each of said sections is confined in a chamber defined, in part, by a microwave permeable window extending across the junction of that section with the third wave guide section.

4. A microwave transmission system comprising three sections of hollow wave guide joined together in a Y configuration and adapted to propagate electromagnetic waves, each of the first and second of said sections of waveguide containing an envelope of ionizable gas at the junction with the third section, each envelope including a microwave permeable window forming a part of the wall of the third section; means for producing a unidirectional magnetic field through the gas in said envelopes in a direction perpendicular to the electric component of the propagated electromagnetic waves, the field having an intensity sufficient to cause cyclotron resonance of electrons in the gas at the frequency of oscillations of electric component of said Waves whereby the Waves incident upon the gas cause electrical breakdown of the gas at the Window and places a short thereacross, and control means for energizing and de-energizing the field producing means for each waveguide section whereby the gas is at cyclotron resonance in one Wave guide section at a time, said energy being blocked from passing through the one section and propagating through the other section.

References Cited in the file of this patent UNITED STATES PATENTS Jenks Aug. 12, 1947 Linder July 8, 1952 Rigrod Aug. 17, 1954 Rines Mar. 19, 1957 Hogan July 2, 1957 Lampert July 8, 1958 Kuecken Dec. 16, 1958 

